Justin Bieber -- 14-Year-Old Threatens To Kill His Ex

Justin Bieber

14-Year-Old Threatens

to KILL Singer's Ex

11/8/2011 12:50 AM PST BY TMZ STAFF

The LAPD got a search warrant against someone who was after Caitlin Beadles --Justin Bieber's former GF ... someone who wanted to skin her alive ... and the perp is a 14-year-old girl.

According to the warrant -- obtained by TMZ -- someone went on tumblr.com, a social media website, using the name KILLSGANDCB -- we're guessing it means kill Selena Gomez and Caitlin Beadles. The perp said she was going to "murder" Beadles and had cameras trained on her 24/7, ominously promising the victim won't know what's coming until she "wakes up on the other side of her bed with a knife."

TMZ has learned ... the suspect is a 14-year-old girl who lives in Kentucky who is developmentally disabled. We're told LAPD detectives contacted her mother, who is now closely monitoring her Internet activities.